This invention comprises an improved process for the preparation of bis(pentachlorocyclopentadienyl), a known chlorocarbon characterized by the empirical formula C.sub.10 Cl.sub.10, known to be useful as an insecticide and as a vulcanizing agent as well as an intermediate in preparation of other useful chlorinated organic chemicals.
Various methods for the preparation of bis(pentachlorocyclopentadienyl) have been described in the chemical literature and have been employed in laboratory or commercial scale preparations. Typically, such methods involve the reductive coupling of hexachlorocyclopentadiene with the aid of a reducing agent such as hydrogen, metallic copper, or cuprous chloride. Generally the preparation is effected in the liquid phase utilizing an organic liquid reaction medium, that is a solvent for the reactants and/or the product.
Cuprous chloride has been found to be a particularly effective agent for the reductive coupling of hexachlorocyclopentadiene. U.S. Pat. No. 2,849,499 discloses a process for the preparation of bis(pentachlorocyclopentadienyl) by reaction of cuprous chloride and hexachlorocyclopentadiene in an aqueous ethanol reaction medium. The impure C.sub.10 Cl.sub.10 product is formed as a sludge which is subsequently separated by decantation of the liquid and then washed first with hydrochloric acid, then twice with ethanol. The sludge is then dissolved in petroleum ether, decolorized twice with activated charcoal and crystallized from solution. Although the process provides a useful method for the preparation of C.sub.10 Cl.sub.10, difficulties are encountered when it is carried out on a commercial scale. In particular, it has been found that the sludge, which comprises the crude C.sub.10 Cl.sub.10 product, must be subjected to multiple purification steps, especially washings which require filtration. The sludge, however, is formed of extremely small particles which tend to pack closely and are filtered with considerable difficulty. It has been found that such difficulties may be avoided by extracting the C.sub.10 Cl.sub.10 product from an alcoholic reaction medium with the aid of a suitable organic solvent, such as toluene, then treating the organic solvent with a weak aqueous hydrochoric acid solution, such as about 0.25 N, to remove unreacted cuprous chloride and subsequently recovering the C.sub.10 Cl.sub.10 product by crystallization from the organic solvent. The hydrochloric acid treatment provides an effective means of purification of the final product by the extraction of unreacted cuprous chloride. Residual HCl may be removed from the organic solvent by extraction with water prior to crystallization of C.sub.10 Cl.sub.10 product. It has been found, however, that the organic solvent phase, containing the dissolved product, is a highly corrosive liquid, typically having a pH in the range of about 3.0-3.5, even after removal of the hydrochloric acid and water washing of the organic solvent phase. The highly corrosive nature of this organic liquor presents problems in further processing, for example, in transferring through pipes, treatment in a crystallizing vessel, and centrifuging to remove the product after crystallization, due to its corrosive action on typical materials of processing equipment. Attempts to increase the pH of the organic solvent liquor and thereby reduce its corrosive activity, through the use of a weaker hydrochloric acid extraction medium, have proven unsuccessful. However, it has been found, surprisingly, that the pH of the organic solvent liquor may be substantially increased, for example to about 6.0 to about 7.0, and its corrosive characteristics decreased accordingly, when a stronger acid, such as about 4.0 N or greater is employed in the hydrochloric acid treatment step.
It will be appreciated that although bis(pentachlorocyclopentadienyl) may be effectively prepared in a known manner, such as by reductive coupling of hexachlorocyclopentadiene with the aid of a reducing agent such as cuprous chloride in an alcoholic medium, a need exists for improved methods for the recovery and purification of the product.
It is an object of this invention to provide an improved process for the preparation of bis(pentachlorocyclopentadienyl). It is a further object to provide an improved method for the extraction and purification of bis(pentachlorocyclopentadienyl) product from an alcohol reaction medium.